Development of the electric vehicle has recently undergone increased activity in an effort to reduce air pollution and conserve fuel resources. A major stumbling block in the development of electric vehicles has been developing a suitable means of supplying power for the electrical drive motors. In most instances, the power has been supplied from a battery source. However, the current battery technology is not capable of supplying a sufficient amount of energy to power the vehicle over extended distances.
Fuel cells have recently been examined as an alternative power source for electrical vehicles. A fuel cell is a demand-type power system in which the fuel cell operates in response to the load imposed across the fuel cell. Typically, a liquid hydrogen-containing fuel (e.g., gasoline, methanol, diesel, naphtha, etc.) once converted to a gaseous stream that contains hydrogen serves as the fuel supply for the fuel cell. Converting the liquid fuel (like methanol or gasoline) to a gas containing hydrogen takes place when the fuel is passed through a fuel reformer. In a fuel reformer, the liquid fuel reacts with steam. The gas formed includes hydrogen gas (20-75% depending on the liquid fuel) and usually contains other passivating gas species such as carbon monoxide, carbon dioxide, methane, water vapor, oxygen, nitrogen, unburned fuel and, in some cases, hydrogen sulfide. An oxidant, usually air, is supplied to the fuel cell to react with the hydrogen gas produced to produce electric current. The electric current can then be drawn on demand in response to loads across the fuel cell to power electrical devices, such as an electric motor of an electric vehicle.
For the fuel cell to generate electric current, however, the hydrogen gas must first be separated from the other gases formed by the fuel reformer. The hydrogen gas atom is very small and it can diffuse through some metals, whereas the other gases have relatively large molecules and so are blocked from passing through. Palladium membranes (i.e., films) are widely used for hydrogen gas separation in the chemical industry. Since palladium is very expensive, however, the membranes can be made less expensive by using a palladium alloy. Metal films can be made by evaporation from a melt under a high vacuum onto a substrate but these films as a rule have a high internal stress, usually compressive. On separation from the substrate, the films spontaneously roll up. In order to make them useful as gas separation membranes, they must be made flat. This involved unrolling the film, holding it down flat and then annealing it, a process having the potential to damage the films and adding to the manufacturing complexity.
I have now found a method of making hydrogen separation membranes by a particular sputtering process yielding flat, thin films. It thus avoids the complexity of the prior art manufacturing process described above.